Proc. Soc. Hort. Sci. 8: 91-95 (1911)
FRUIT-BUD FORMATION AND DEVELOPMENT*
A. W. DRINKARD, JR.,
Experiment Station, Blacksburg, Va.

*Note: For a more detailed discussion of this subject, the reader is referred to an article on the same subject in the Annual Report of the Va. Agrl. Expt. Sta. for 1909 and 1910, p. 159-205.

In 1908 the writer began a series of morphological studies of fruit-buds at the Virginia Agricultural Experiment Station under the direction of Mr. H. L. Price, the station horticulturist. The purpose of these studies was to gain certain knowledge about fruit-buds preparatory to a study of the effect of soil environment on fruit-bud formation which Mr. Price had projected. The plan of the morphological studies was to determine the period of fruit-bud differentiation, the time at which the floral parts are formed, and the period at which the fruit-bud is complete as regards its morphological development. The apple, pear, peach, Plum and cherry were studied.

Our observations extended over two years. Buds were studied from the latter part of June to the blooming period the following spring. Particular trees in the orchard were chosen for the observations. Samples of buds were taken from the trees at intervals of a week or ten days throughout the period of observation. These buds were killed and fixed immediately. Some of the outer bud-scales were removed in order to facilitate penetration by the fixing agent. Several fixing agents were tested in the course of the work; Flemming's Waeker Fluid and Gilson's Mixture proved most satisfactory for our purposes. The buds were then washed and dehydrated, infiltrated with paraffin and imbedded in paraffin. In some cases a combination of the paraffin and collodion methods was used with good results where hard material had to be sectioned. The next step was to section the buds, after which the sections were stained and mounted. Staining was done in some cases with Flemming's triple stain; in other cases Delafield's haematoxylin was sufficient to bring out the structures. The sections were mounted in Canada balsam. Microphotographs were made from the microscopic preparation covering the main points in the development of the fruit-bud.

The Apple.—Several varieties were observed in a preliminary way, but the studies were concerned mainly with the variety Oldenburg. In the latter part of June there were slight lobulations on the growing axis of some of the buds, indicating the initial differentiation of buds into fruit-buds. By the middle of July the fruit-buds in the bud clusters had become distinct and there was some indication of flower-parts in the fruit-buds. At the end of the growing season all the flower-parts showed clearly, although they were not complete. Thus the buds entered the resting stage. During the winter season there were some slight changes going on. The anthers enlarged appreciably during February. Early in March ovules appeared in the cells of the ovaries. By the first of April all flower-parts were complete. Other varieties were compared with the development of fruit-buds in Oldenburg, and no great differences were observed.

The Pear.—Our observations were confined to one variety, the Kieffer. Development of the fruit-bud is quite similar to the development of the fruit-bud in the apple, except the formation of the fruit-bud in the pear takes place a little later, about the middle of July, and the flower parts are complete and ready to unfold somewhat earlier in the Spring. The stage of development at which the pear buds entered the winter season was very similar to the same stage for the apple. Ovogenesis and spermatogenesis occurred in February. Early in March, the fruit-buds were complete and ready to unfold.

The Peach.—Fruit-buds of the variety Luster were studied. There was no differentiation of buds into fruit-buds until the end of the first week in July, at which time a slight lobulation on the growing axis made its appearance. During the month of August, development proceeded rapidly and all flower-parts had appeared by the middle of September. The flower-parts were well advanced when the winter season came. On December 19, tetrad formation in the pollen-mother cells of peach anthers was observed, and about the same time the ovule could be seen in the ovarian cavity. It is remarkable too that division of pollen-mother cells, which gives rise to microspores and these in turn give rise to pollen cells, continued over a long period of time. Early in March there was a marked increase in the size of the fruit-buds and the flowers were ready to unfold by the middle of the month.

The Plum.—A number of varieties were observed; Whittaker, a variety which belongs to the Wild Goose group, was studied in detail. There is considerable difference in the time of formation and development of fruit-buds in different groups of plums. Whitaker showed some indication of fruit-bud differentiation the latter part of July, although differentiation was not general until the first week in September. And so this variety entered the winter season with its flower-parts in the initial stages. But during the latter part of January and early in February, development of the flower parts was very rapid, and early in March the flowers were complete.

Several varieties belonging to the Japanese group (Prunus triflora) were observed, and it was found that differentiation of the buds occurred during the second week in July, and the fruit-buds were distinct the first week in August. In the variety Abundance, tetrad formation in the pollen-mother cells was observed in one instance on September 1. Hale plum showed all flower-parts complete with anther sacks showing microspores at the middle of December. Generally varieties in this group show the pollen-mother cells in the resting stage during the early part of January.

The Cherry.—The variety Louis Phillippe, one of the sour cherries (Prunus cerasus) was studied. In the latter part of June slight lobulations in the crown of the growing axis were discerned. During the month of July the fruit-buds became clearly differentiated. During August the flower-parts appeared, and at the end of the growing season the flower-parts were well advanced in development; in fact almost as much developed as those of the peach for the same season. In February and March important development took place in the essential parts of the flower. Tetrad formation in the pollen-mother cells showed by the middle of March and at the first of April the flowers were ready to unfold.

Differentiation of buds and the formation of fruit-buds takes place during the latter part of June or early in July, depending upon seasonal conditions and the species in question. Important development takes place in the fruit-buds before the resting stage is reached. In the practice of orchard operations designed to influence or control fruit-bud formation, it appears that such operations should be more effective during the early spring and summer of the year in which the fruit-buds are formed.

THE PRESIDENT: Do you often find two ovules in the plum?

MR. DRINKARD: No, it is very exceptional.

A MEMBER: Some of us who come from sections farther north think of the branches as being frozen during the winter months. Were those with which you worked frozen?

MR. DRINKARD: So far as I know they were not.

PROFESSOR MACOUN: I would like to know whether Mr. Drinkard has found any relation between the time of rapid growth of the different kinds of fruit trees and the beginning of the formation of the fruit-bud. It has been a little theory of mine, not borne out by any actual experiments, that the formation of the fruit-buds is due in part to the retardation of the elaborated sap in the upper part of the tree. For instance, we find that ringing a tree will cause the development of fruit-buds. We find also that a branch which is partly broken will fruit perhaps two or three years before the other part of the tree. We find that a tree which is top-grafted will fruit sooner than a tree which is not. It strikes me that there may be some relation between the retardation of the elaborated sap by ringing, breaking or grafting and the formation of fruit-buds. It has also been noted by fruit-growers that a dry season is followed by a good crop of fruit the next year.

MR. DRINKARD: I am sure I cannot say whether the correlation will hold completely or not. As a matter of fact, the main part of the vegetative growth has been completed when the differentiation in the buds takes place, but I do not think it will hold all the way through. For instance, you will notice a very considerable difference in the development of different varieties of plums. The vegetative growth in these plums was completed at approximately the same time, and yet there was a very material difference in the time of bud formation. It is certainly a fact that retardation of sap flow will cause the formation of fruit-buds. For instance, Reed, at the Virginia Station, has reported a case where a blight of some kind had caused the blooming of certain trees in the fall of the year, and he attributed the premature blooming to the influence of this blight. There is no doubt that the cessation of vegetative growth has a marked influence on fruit-bud formation, but to what extent they are correlated I cannot say.

THE PRESIDENT: Have you found any difference as to the development of the fruit-buds on different portions of the twig, that is, at the base, middle, or outer end, and what plan did you follow in making the selection of the buds?

MR. DRINKARD: I did not find any very great difference. For instance, we ordinarily think that the buds on the lower part of a twig of the peach where the fruit-bud is always formed on the current year's growth, would be more advanced than those above; but as a matter of fact that does not hold altogether. It may be true, but it does not necessarily follow, and you will frequently find buds half, way up. For instance, suppose you have a twig which has grown twenty-four or thirty inches in a season; half way up that distance you may find buds more developed than near the base. I think, therefore, the location of any given bud will not determine the differentiation of that bud, nor the beginning of the development of the flower-parts in the individual fruit-buds.

PROFESSOR BLAKE: I think that where a peach twig grows rather rapidly, it is a common occurrence for the strongest fruit-buds to be about in the center of the season's growth; near the base and also near the top they are smaller. I have often observed in such varieties as Reeves that the two outside buds in the group of three are often leaf buds near the top and fruit-buds in the middle of the growth.

MR. DRINKARD: I had not made that observation.

THE PRESIDENT: If there is no further discussion I will call for Professor Blake's paper on Factors Which Affect the Blooming and Ripening Period of Peaches.